Growing Pains
by shanghaigirl2010
Summary: When Kurotsuchi attempts to further Aizen's research, Grimmjow and Ichigo are left to care for a shinigami-arrancar child. Hijinks ensue. Takes place in a canon-divergent AU sometime after the Thousand Year Blood War arc. Slightly fluffy crack/dry humor with a generous sprinkling of bleachlists references.
1. Chapter 1

**A/N:** Originally based on a fanart I drew for bleachlists on tumblr, which led to me creating a backstory that I decided I wanted to continue.

* * *

"N-No! You can't experiment on him!" Ichigo shouted.

"What the hell are you yelling about, Kurosaki?" Grimmjow asked, appearing almost instantly beside Ichigo, familiar adrenaline rush coursing through him. It had been months since the war with the Quincy had ended, so there wasn't much going on that would make Ichigo panic like that. Grimmjow hoped it was an enemy, preferably a strong one.

"Shh, it's okay. I'll protect you," Ichigo said soothingly to the small bundle cradled in his arms. Frightened whimpers gradually grew fainter, before being replaced by barely audible breathing.

Grimmjow glanced over Ichigo's shoulder, eyes widening as he saw a sleeping infant burritoed inside a black blanket. Soul Society might have had plenty of dead children wandering around, but newborn babies were rare. "Where the fuck did that come from?"

Ichigo glanced up at him, his usual scowling expression replaced with uncertainty. "Well, technically… us."

"Did I hit your head too hard during our last fight?" Grimmjow asked, eyes narrowing. Unless the rules of human physiology had recently changed, he was fairly certain Ichigo was not capable of reproduction. Grimmjow also hadn't done anything remotely sexual with the orange-haired brat, but that was a dangerous train of thought to explore. He was more comfortable thinking about beating Ichigo, as opposed to beating off on Ichigo.

"What Kurosaki means is that this specimen is a result of your combined genetic material," a clinical voice said, from just behind Grimmjow.

Grimmjow tensed and immediately whirled around to find himself facing a smug-looking Kurotsuchi. The shinigami version of Szayel, he recalled, and not any less morally questionable or creepy. He automatically shifted into a defensive stance. "What did you just say?"

"Oh, I'm sorry. Did I use too many big words there, Espada?" Kurotsuchi asked, eyes glinting with amusement as Grimmjow snarled at him. "Let me make it simpler for you. When two people make a baby -"

"I fuckin' got that part," Grimmjow spat. "I don't get why."

"That should be obvious enough. Shinigami are made stronger by blurring the line between them and hollows, and vice versa. Therefore, the most effective combination would be a specially engineered hybrid, much like Kurosaki," Kurotsuchi explained, gesturing towards Ichigo. "He is our strongest shinigami, and you are the strongest existing arrancar. You were both prime candidates."

Grimmjow took a few seconds to let that information sink in. So the kid was his. His and Ichigo's. _Well, shit_. That was a lot to deal with for someone who'd been dead and alone for the better part of several decades.

"Now, if you could kindly return the results of my research to me, Kurosaki," the scientist continued, taking a step forward.

Grimmjow found himself blocking the way, a growl building in the back of his throat as he glowered at Kurotsuchi. "Fuck off. He's not yours." His hand twitched towards Pantera, eager for an excuse to attack the puppet-faced shinigami.

Kurotsuchi tilted his head to the side. "I was not expecting you of all people to object, Jaegerjaquez."

Ichigo seemed to share that sentiment, staring at Grimmjow with unguarded surprise.

"Shut up," Grimmjow snapped, even as he wondered why he felt the need to intervene. He was the espada of destruction. He shouldn't give a damn what Kurotsuchi did with that tiny half-human-hollow-whatever. Whether it lived or died made no difference to him. Yet, he was inexplicably bound to it. Much like how he'd claimed Ichigo as his prey, but not quite the same.

"My, my, what's this?" A familiar figure in a green and white striped hat and clogs appeared from just behind Ichigo.

"Urahara," Ichigo acknowledged, with a mixture of both relief and vague suspicion. The baby clutched to his chest stirred slightly.

Urahara leaned in, his inquisitive expression turning to one of surprise, as he noticed the distinctive red markings along the infant's cheek. "Oh. This is..."

"A sample of my precious work," Kurotsuchi offered.

"This reiatsu," Urahara murmured, realization flickering in his shadowed eyes. "It's remarkably strong for a newborn's," he said, almost conversationally. "In fact, something about it seems familiar."

"There is no need to pretend," Kurotsuchi quipped dryly. "You must have noticed my recent experiments, _Captain_."

Urahara smiled. "What do you mean? I'm only an innocent, harmless shopkeeper." When his comment was met with silence, he shook his head and sighed. "It's just not the same."

"Yeah, that line doesn't work anymore," Ichigo agreed. Urahara had been promptly reinstated as Captain of the Twelfth Division after the war, much to his chagrin and the relief of its squad members.

Urahara's shoulders drooped slightly, before he straightened back up. "Well, it can't be helped. Kurotsuchi, I've got a new assignment for you back at the lab."

Kurotsuchi's eyes narrowed. "But what about -"

Urahara cut him off by wagging a finger at him admonishingly. "Ah, ah. I'm sure Ichigo is more than qualified to track and monitor your little project for you. It's about time he did some work around here."

"Hey!" Ichigo protested, even as Urahara gave him a sly wink. "I saved the universe twice, and this is the thanks I get," he mumbled under his breath.

"I expect detailed reports by the end of each week," Kurotsuchi said, before following Urahara back to their barracks.

Ichigo rubbed his temple and groaned. "Great. More paperwork." Being a captain really wasn't as fun as he'd imagined it to be. There were so many tedious rules and regulations to follow, members to be trained, missions to be delegated. No wonder Byakuya acted like he had a sword up his ass all the time.

"Tch, I didn't even get to fight the bastard," Grimmjow complained loudly, sauntering back over. At the sound of his voice, the baby yawned and blinked sleepily, before staring at him with a pair of startlingly bright blue eyes. "I know what you're thinking, Kurosaki," Grimmjow warned. "Don't you fucking say it."

Ichigo grinned, amusement tempered with a certain softness in his gaze. "He has your eyes."

"Bite me, shinigami," Grimmjow said, no fire behind his words. He watched with fascination as the baby reached out with its tiny hands and patted Ichigo's cheek.

"What should we name him?" Ichigo asked, practically glowing with affection.

Grimmjow snorted. Ichigo was such a pushover when it came to dealing with children. First Nel, now this. It was hard to believe that the remarkably strong warrior Grimmjow had fought nearly to the death was capable of being just as fiercely kind and caring. It still confused him at times. In Grimmjow's experience, you couldn't be both. "As if I care. Name him whatever the fuck you want," he said dismissively.

Ichigo's eyes caught on a small tag sewn into the lower left corner of the blanket, the number nine. "Kyu," he said, testing it out. "I'll name him Kyu."

Grimmjow shook his head. "I can't believe you're gonna name him after a number."

"You said you didn't care," Ichigo retorted.

"Yeah, well, it's still a stupid name."

Ichigo glared at him. "Do you have any better ideas?"

A pause. "No," Grimmjow reluctantly admitted.

"Then it's settled. Come on, Kyu. Let's introduce you to the other captains."

* * *

"Oh my gosh, he's so adorable!" Matsumoto squealed, nearly smothering the baby with her breasts. "It's like a little version of you! Except, um, half-arrancar," she said, with a nod to Grimmjow.

"I can't believe I have a grandson already!" Isshin exclaimed with glee, snatching Kyu from his lieutenant's embrace and lifting him up. "Though I'm not sure about your choice of partner," he added, sizing up Grimmjow. "Didn't he try to kill you at least three times?"

Ichigo's cheeks flushed a dark pink as he took a dizzy-looking Kyu back. "D-Dad! We're not together like that! Besides, Grimmjow's an honorary member of Squad 11 now." Ichigo would've offered him a lieutenant position, but Grimmjow would never have agreed to work under him.

"I trust you, son," Isshin said, letting his serious side show through. "Now, I have a meeting to get to. I'll be seeing you around." He gave Ichigo a firm pat on the shoulder before leaving.

Ichigo sighed. It had taken some time to get used to the idea of his goofy, unpredictable dad as a shinigami, and even now, it was strange working with him on a daily basis. They were considered equals as captains of the Tenth and Eleventh, but Ichigo always felt like a kid around him.

"What a pleasant surprise," a dark, silky voice remarked, cutting through his thoughts. "I suppose congratulations are in order."

"Not going to explain how it was all part of your plan?" Ichigo asked, meeting Aizen's gaze as he stepped into the now silent room. Grimmjow bristled beside him, distinctly uncomfortable sharing the same space as his former boss, and Matsumoto didn't bother to hide the furious glare she directed at him. Ichigo knew she still blamed the man for Gin's death.

Aizen had left a lot of bad blood in the wake of his betrayal, but his help during the last war had been invaluable. Considering the significant losses the Gotei 13 had suffered and the shortage of qualified applicants, Kyoraku had offered him a captain's position. To everyone's surprise, and Hinamori's absolute horror, Aizen accepted. He was currently in charge of Squad 3, Gin's old division. Whether that was a tribute to or mockery of his memory was anyone's guess.

"Of course. I faked my own death and created an army to take down the Soul King, only to purposely lose, just so I would one day witness the birth of a rare shinigami-arrancar hybrid," Aizen replied, tone heavy with sarcasm, though there was a playful lilt to his words. "Everything has gone according to my plan, Kurosaki Ichigo."

"Please, no more monologues," Ichigo said, cracking a wry smile. Despite everything that had happened, Ichigo was arguably on the best terms with Aizen. True, Aizen had once tried killing him and his loved ones, but so had half the seated officers in Soul Society. Ichigo wasn't one to hold grudges. Besides, fighting was a tried and true way of making new friends.

"Do you want to hold him?" Ichigo asked politely. Grimmjow stared at him like he was offering to throw the infant to a pack of rabid Menos Grande.

Aizen raised an eyebrow at the offer, but nodded. "I may as well." He accepted the baby from Ichigo, holding him at a slight distance, as though he were liable to explode.

Curious blue eyes regarded Aizen carefully for a few seconds. "He is rather cute," Aizen admitted, bringing him closer. He stiffened as Kyu grabbed the stray lock of hair hanging in front of his face, and Grimmjow snickered at Aizen's look of mild irritation.

"Is something burning?" Matsumoto asked, sniffing the air. Almost immediately after her question, there was a strange zipping sound accompanied by a flash of light. Ichigo turned to look at Kyu, who gurgled happily and released the singed remnants of Aizen's hairlock before clapping his hands together.

"Was that - a cero?" Ichigo asked, bursting into laughter at Aizen's completely shell-shocked expression.

"I think I like this kid," Grimmjow said, with an approving grin.

"Perhaps I judged too early," Aizen remarked, eyeing Kyu with thinly veiled distrust as he handed him back to Ichigo. "No doubt he will grow up to be just as troublesome as his predecessors were." He turned to leave, only to be blindsided by Rukia rushing into the room, followed closely by Renji.

"Ichigo! Why didn't you tell me first, idiot?" Rukia demanded, arms crossed.

"Tell you what?" Ichigo asked, looking confused. "It's not like I was expecting this!"

"Well, you could've at least said something about Grimmjow," Renji interjected, shooting Ichigo a look of disappointment. "Come on, man. Aren't we friends?"

"I-It's not like that! He isn't - "

"Awww! He's even cuter than Chappy!" Rukia gushed, gently squishing Kyu's cheeks, before noticing Aizen quietly dusting himself off. "What's _he_ doing here?" she asked, tone suddenly ice cold. Rukia, for obvious personal reasons, was one of the many who had yet to forgive Aizen.

"I was just leaving," Aizen answered calmly. "Captain Kuchiki." After Ukitake's all-or-nothing attempt at saving the Soul King, his health had degraded beyond the point of being able to hold a captain's position, so Rukia had succeeded him, despite her brother's initial protests.

Rukia scowled at him until he exited. "I can't believe Captain Kyouraku asked that traitor to stay."

It had been a delicate situation. Kyouraku was reluctant to throw the man back in prison, though Central 46 and a few others certainly pushed for it. Ultimately, Ichigo's insistence on leniency had forced them to back down. Aizen had done some terrible things, but he _had_ been right in the end.

"I mean, would you want Aizen wandering around unattended?" Ichigo asked, careful not to sound too defensive. "It makes more sense to keep him here."

"You know what they say about keeping friends close and enemies closer," Matsumoto chimed in, tossing a suggestive look towards Grimmjow. She sighed dramatically. "I never thought our innocent little Ichigo would grow up so fast."

"For the last time, Grimmjow is _not_ an enemy, and we are _not_ together!" Ichigo insisted, frustration finally getting the better of him. "Did you think we just hooked up, and I magically had a kid?"

"Well, weirder things have happened around here," Renji admitted, scratching his head.

"I always thought he had a thing for you," Matsumoto said innocently. "Sneaking into the Human World just to find you, screaming your name when you were on video call with Urahara -"

Grimmjow growled. "Who the hell told you that? I was _not_ \- "

"Wait, isn't that why you joined Urahara in the first place? To see Ichigo again?" Renji cut in. Rukia nodded thoughtfully.

"No! I just wanted to fight him!" Grimmjow insisted, while Ichigo hid an amused smile. It wasn't often that the espada got caught up in petty arguments. Grimmjow usually preferred to settle things with his sword, roughly and violently. Er. Ichigo blinked and mentally revised that thought.

"'Fight' him," Matsumoto repeated, with a wink.

"Um. Maybe we should go… Kyu looks a bit sleepy," Ichigo said, at Grimmjow's furious expression. It wouldn't be good if he decided to start a fight after all.

"Stupid shinigami," Grimmjow muttered under his breath as they hastily left the room. "Almost makes me wish I was still working for Aizen."

"He let Tosen cut off your arm."

"I said _almost_."


	2. Chapter 2

"Ichigo, may I have a word?" Urahara asked, from just outside Ichigo's office.

Ichigo looked up and set aside the list of updated regulations he'd been mindlessly scanning over. "Sure. What's up?"

"Regarding the child. I recommend you raise him in the human world," Urahara said, taking a seat in front of Ichigo's desk.

"Why?" Ichigo asked, surprised. "He's more hollow than human." Ichigo had taken the time to map out his own bloodline and use it to sort out exactly what Kyu was - slightly less than a quarter Quincy, some fraction pure hollow, a quarter shinigami, and half arrancar.

"He'll be safer there," Urahara replied, voice lowering.

"What? But Kurotsuchi - "

Urahara shook his head. "He's not the problem." He looked at Ichigo with an uncharacteristically serious expression, before sighing. "You remember how Soul Society saw you at first, Ichigo. As a threat. Now, you've been their ally for quite some time, and they've come to accept you as one of their own," Urahara said. "That does not apply to this infant."

"But he's mine," Ichigo said, feeling strange making that claim. After all, he had only just turned eighteen. Sure, he'd technically died twice already and was currently in charge of a squad of soul reapers, but taking care of a kid was a lot of responsibility.

"Yes, but he's also the offspring of a former Espada," Urahara pointed out. "Although Grimmjow is allowed to reside within Soul Society, he is still kept under close watch, just as Aizen is." He spoke the last few words with a faint distaste.

Ichigo had a feeling that Urahara still suspected the former traitor of _something_ , though Aizen had been on exceptional behavior since rejoining Soul Society. It wasn't an unreasonable belief. The possibility of Aizen's shikai taking effect was a perpetual threat, despite the fact that he had agreed to have Kyouka Suigetsu kept in Kyoraku's office as a precaution. As one of the few officers immune, Ichigo was in charge of keeping tabs on him to make sure he wasn't secretly turning people into hollows.

"More importantly, your celebrity status could negatively affect his upbringing," Urahara continued. "It would be best to let him mature a little before telling him about your colorful past."

Ichigo nodded. "Of course." He definitely wasn't going to make the same mistake Isshin did. As soon as Kyu was old enough to understand, Ichigo was going to sit him down and explain that it was perfectly normal to see dead people and maybe the occasional freakish monster, and there was no need to worry, because his father would do everything to protect him until he could fight for himself.

Still, it surprised him that Urahara was being so considerate of Kyu's psychological well-being. He had never seemed to give much thought to Ichigo's, what with the whole throwing him into a pit to either die or develop hollow powers, and sending him off to fight Aizen without any warning of what to expect. Ichigo didn't think about these things often, but they occasionally surfaced in his mind and left a sort of confused disappointment in their wake.

"I would advise Grimmjow to do the same." Urahara scratched his head. "To be honest, I'm not sure Grimmjow would make a very good parent."

"I think he could be," Ichigo said, somewhat defensively. It was hard to imagine, but Ichigo believed the espada had a softer side hidden beneath his brash and bloodthirsty demeanor. Maybe this situation would be enough to draw it out. At that, his eyes widened in realization. "Wait, how am I supposed to tell him? Would he even want to live in the human world with me?" Ichigo wasn't even sure if Grimmjow could handle something like that. He cringed as he imagined Grimmjow shouting at innocent passersby on the street, punching through walls, and generally living up to his destructive title.

Urahara shrugged. "I have no idea. I'm sure you'll think of something."

"What would we even do there?" Ichigo frowned, his train of thought spawning new concerns. "I'm not sure I could get a job. I barely graduated high school." He'd only been back in his physical body once in the past few months, and that had been to say goodbye to his friends and his sisters. He didn't even want to think about what Kon was doing with it while he was away, but he could bet he wasn't attending college.

"Ah, there's no need to worry about that," Urahara said, snapping his fan open with a flourish. "I've been looking for someone to help run my candy store, and you're perfect for the job!"

"It's still open?" Ichigo asked, raising an eyebrow. He had assumed Urahara had sold it off before leaving. There wasn't really a need for stocking questionable shinigami merchandise when he had full access to the Twelfth Division labs.

"Temporarily closed," Urahara corrected. "But it won't be for long!" He flashed Ichigo a gleeful grin, something that automatically made Ichigo a bit nervous, as it usually signalled some impending, life-threatening circumstance.

"Let me talk to Grimmjow about it first," Ichigo said warily. "And I'll probably need Kyoraku's approval too." Oh god, there was probably going to be a fuckton of paperwork to fill out. Did Soul Society even offer paternity leave? Though, he supposed it didn't matter, considering they didn't pay him. He just had to make sure they didn't order him to be executed through some misunderstanding. That would be awkward.

* * *

"Do I get to fight you?" Grimmjow asked, arms crossed, after listening to Ichigo's five-minute speech about how their child deserved to be raised in a loving family environment with both DNA contributors present. It hadn't quite been on the same level as Aizen's monologues, but it came pretty close.

"Uh, sure, I guess," Ichigo said.

"Then yeah, I'll go."

Ichigo blinked. "Really?" He hadn't expected it to be _that_ easy.

"What, is there a catch or something?" Grimmjow eyed Ichigo with suspicion. He'd learned to be more careful ever since he found out what his contract with Urahara had actually stated. It had been long voided, but not before he was made aware that he had technically agreed to "provide personal services for interested customers", whatever the fuck that meant. Based on the way that Matsumoto chick had giggled, he guessed it wasn't good.

"Well, you're going to have to help me out with Urahara's shop," Ichigo said. "And it'd be nice if you kept an eye on Kyu every once in a while."

Grimmjow shrugged. "Sounds good to me." He wondered why Ichigo had made this out to be such a big deal. Raising a kid was just like having a fraccion, right? Except related to him. And probably more resistant to getting eaten.

"You'll, um, have to wear a gigai too," Ichigo added, hoping that Grimmjow wouldn't be put off.

Grimmjow's eyes narrowed. "A gigai?" He remembered Urahara mentioning something like that before. Some sort of artificial body? It sounded kinda sketchy, but not dangerous in an interesting way, so he hadn't paid much attention.

"It's like... a person-shaped container for your soul," Ichigo improvised. Good enough. "It would be weird if normal humans couldn't see you." His brightly colored hair already drew enough negative attention from others. He didn't need to be known as that one delinquent single dad who talked to himself.

"Sure, whatever," Grimmjow said. As long as he could use his powers and fight Ichigo, everything was perfectly fine. He'd been getting sick of hanging around all these shinigami anyway. "When are we leaving?"

"I'm not sure." Based on Rukia's previous ordeal, Ichigo guessed his request for leave could take anywhere from hours to months to get approval. "Depends on what Central 46 says."

"Those old fuckwits?" Grimmjow asked, with a look of extreme disdain.

His hatred was understandable. Once upon a time, Central 46 had really, really, pretty-please-with-a-hogyoku-on-top wanted to get rid of Grimmjow, both because of his hollow origins and his previous affiliation with the guy who wanted to take over the universe. The whole espada of destruction thing also raised a lot of red flags.

As with Aizen's case, it had taken Ichigo's mediation to change their minds, which was really more of an Ulquiorra-type ultimatum, along the lines of "If you try to kill him, I quit." Emphasis on _try_ , because no one had ever succeeded in eliminating any of Ichigo's friends. So, at the risk of losing their strongest ally, Central 46 had reluctantly given in. He often debated whether they were threatened by his power, grateful for his help, or plotting to secretly assassinate him someday. Or some combination of all three.

"It'll probably take a few days," Ichigo assured. "So maybe next week or sometime later."

Grimmjow stared at him with distrust. "Last time, when you said I could fight you "sometime later," I had to wait _years_."

"It was a year and a half," Ichigo corrected.

"Well, it felt like years!"

Grimmjow's expression veered dangerously close to a pout, and Ichigo had to hide a smile at the sight. "Missed me that much?" he teased.

"The only thing I missed was kicking your ass!" At that, Grimmjow reached for his zanpakuto, baring his teeth in his trademark grin. "Bring it, Kurosaki."

Ichigo's eyes widened. "But I have a meeting in fifteen minutes."

"Fuck that. You and me. Right now." Grimmjow drew his sword in a smooth motion, the flash of excitement in his eyes matching the glint of his blade against the mid-afternoon sunlight.

"Grimmjow, I can't - "

"Come on, _Captain_. Unless you're afraid you can't hold on to your position," Grimmjow jeered. It was an empty jab, as much as it stung his pride to admit. Though they might've been evenly matched a year ago, these days, Ichigo won more than half their fights. But that didn't make their matches any less enjoyable. If anything, it made his occasional victories even sweeter.

Ichigo looked torn between his love for a good fight and his responsibilities, seconds from giving in to some quick sparring, when a smooth voice drew his attention.

"Am I interrupting something?"

Both Ichigo and Grimmjow turned to see Aizen watching them with mild interest.

"Yes," Grimmjow spat, overlapping with Ichigo's equally hurried denial. _Damn it._ Even now, he was getting cockblocked. "Mind your own fucking business, Aizen," he growled, purposely leaving off any sort of title, smug in the knowledge that his insolence would go unpunished. Aizen was definitely still capable of using his spiritual pressure to slam him to the ground, but he wouldn't dare in front of Ichigo.

"Still haven't fixed that filthy mouth of yours, I see," Aizen remarked, with more amusement than disapproval.

Grimmjow's eyes narrowed, some animalistic part of him considering just charging the other man, even as his logical side told him that it would be a terrible idea and likely end in humiliating defeat. Though Aizen was technically unarmed, that did not make him any less formidable of an opponent.

Ichigo shot Grimmjow a warning look, as if sensing his thoughts. "Aizen. Did you need me for something?" he asked.

"No, nothing important," Aizen said dismissively, eyes flickering to Grimmjow. "I'll see you at the meeting, Captain Kurosaki." With that, he turned and walked away, leaving just as suddenly as he had arrived.

"Creepy bastard," Grimmjow muttered, as soon as Aizen was out of earshot. "Always sneaking around like that." He looked back towards Ichigo, only to find the other man running away in the opposite direction. "Oi! Come back here!" he yelled, giving chase.


End file.
